Most flat glass is today manufactured by the float production method, developed in England in the late 1950s. The float process has virtually eliminated production methods, such as sheet or plate glass manufacture. Float glass is made in a float tank, which once placed into operation, is operational 24 hours per day until it is refurbished, which can occur up to a decade later.
The manufacturing process can be broken down into five main stages. The first is the batching of raw materials. In the case of soda lime glass, the main ingredients are silica sand, calcium oxide, soda and magnesium. The components are weighed and mixed into batches to which recycled glass (cullet) is added.
Next the raw materials are loaded into a furnace where they become molten at a temperature of approximately 1550.degree. C. The molten glass "floated" onto a bath of molten tin at a temperature of about 1000.degree. C., forming a large ribbon. The glass is highly viscous and the tin is very fluid. As such, the floating glass does not mix with the tin and the contact surface between the two materials is perfectly flat.
On leaving the bath of molten tin, the glass, now at a temperature of about 600.degree. C., has sufficiently cooled to pass into an annealing chamber called a lehr. Within the lehr, the cooling rate of the ribbon is controlled to obtain annealed material free of internal mechanical stresses, enabling the ribbon to be cut and worked. After cooling, the glass undergoes quality checks prior to being cut into sheets, which are stacked or stored for transport. A conventional apparatus for producing float glass is illustrated and described in U.S. Pat. No. 3,083,551.
The newly formed ribbon of glass is advanced through the annealing lehr on driven conveyor rolls which draw the glass ribbon along the molten metal bath. The glass, which is in a plastic condition as it enters the lehr, is subject to a defect known in the art as "roll print." Roll print is characterized by distortions or imperfections such as random surface rub marks, fissures, and, in some cases, fractures.
Roll print is often caused by the mechanical contact of the glass ribbon with the lehr conveyor rolls. In time, the rolls tend to accumulate uneven and crusty surface deposits which imprint on the soft undersurface of the newly formed glass ribbon. It is believed that such deposits are formed on the rollers through the collection of oxides, as well as dross and other foreign matter adhering to the soft undersurface of the glass ribbon.
Attempts have been made in the past to remove these crusty deposits from the surfaces of such conveyor rolls in situ. U.S. Pat. No. 3,337,320 teaches a method of removing deposits from the surfaces of lehr rolls by steam. U.S. Pat. No. 3,481,727 discloses the use of a reciprocating abrasive tool for abrading and brushing the crusty deposits from the surfaces of conveyor rolls.
U.S. Pat. No. 4,042,364, describes a system to remove impacted deposits from the surfaces of moderately encrusted conveyor rolls using a floor-mounted mechanism. However, the apparatus is complex and of heavy construction to provide the brushing forces required to remove tenacious deposits from heavily encrusted conveyor rolls. Additionally, since the space between the lehr rolls and the floor is of a very limited height, use of such an approach is not feasible in all circumstances.
U.S. Pat. No. 4,208,754 addresses this problem by providing an off-line lehr conveyor roll cleaning apparatus. The system comprises a mobile lower main unit having a pair of rotating roll supporting stations, one at either of its ends, and an upper removable unit which includes a longitudinally reciprocal, rotary brushing wheel. The lower main unit may include heater devices for heating the roll while it is being cleaned in order to maintain its temperature so that the surface encrustations can better be removed and/or it can be reinstalled in an operating lehr immediately after being cleaned without the usual preheating step.
Although removal of the conveyor roll may ease maintenance of the roll itself, roll removal is not recommended in most cases. Due to its maturity, the manufacture of float glass is a highly competitive process, such that down time must be monitored carefully to maximize throughput and maintain acceptable levels of profitability. Removal of the rollers, though conducive to a high level of surface cleaning, often leads to excessive disruption. The still remains, therefore, for a simple but effective in situ conveyor roll cleaning method and apparatus.